Method for increasing the strength of a multiple-layer shell type cylindrical presssure vessel

ABSTRACT

A method for improving or increasing the strength of a multiplelayer shell type cylindrical pressure vessel, the shell of which, due to the inner pressure applied, tends to present a sine-curve like bending behavior, i.e., a series of continuous curvatures appearing alternately on the opposite sides in the shell, with respect to the neutral line of the bending curvatures. (The intersecting point of such a curvature of bending with said neutral line will be referred to as a bending-direction transition point hereinafter.) The method provides at least one of such transition points a circumferential weld extending throughout the wall thickness of the shell or provides a solid wall type shell section extending from the joint between the head plate and the shell to the first transition point or provides a solid wall type shell section between the two transition points, thereby increasing the bending rigidity or strength at such transition points for the cylindrical pressure vessel.

United States Patent 1 Yamauchi 1 Mar. 18, 1975 METHOD FOR INCREASING THE STRENGTH OF A MULTIPLE-LAYER SHELL TYPE CYLINDRICAL PRESSSURE VESSEL [75] inventor: Takeshi Yamauchi, Kobe, Japan [73] Assignee: Kobe Steel, Ltd., Kobe, Japan [22] Filed: Aug. 17, 1973 [21] Appl. No.: 389,262

[30] Foreign Application Priority Data Primary Examiner-C. W. Lanham Assistant Examiner-Victor A. DiPalma Attorney, Agent, or FirmOblon, Fisher, Spivak, McClelland & Maier [57] ABSTRACT A method for improving or increasing the strength of a multiple-layer shell type cylindrical pressure vessel, the shell of which, due to the inner pressure applied, tends to present a sine-curve like bending behavior, i.e., a series of continuous curvatures appearing alternately on the opposite sides in the shell, with respect to the neutral line of the bending curvatures. (The intersecting point of such a curvature of bending with said neutral line will be referred to as a bendingdirection transition point hereinafter.) The method provides at least one of such transition points a circumferential weld extending throughout the wall thickness of the shell or provides a solid wall type shell section extending from the joint between the head plate and the shell to the first transition point or pro vides a solid wall type shell section between the two transition points, thereby increasing the bending rigidity or strength at such transition points for the cyli11- drical pressure vessel.

3 Claims, 6 Drawing Figures IKJEHTEU 185575 SHLU 1 OF 3 FIGI FIG. 5

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METHOD FOR INCREASING THE STRENGTH OF A MULTIPLE-LAYER SHELL TYPE CYLINDRICAL PRESSSURE VESSEL BACKGROUND OF THE INVENTION l. Field of the Invention This invention relates to a method for increasing the strength of a multiple-layer shell type cylindrical pressure vessel, and more particularly to a multiple-layer shell type cylindrical pressure vessel, the shell of which tends to present a sine-curve like bending behavior due to the bending force created at or in the vicinity of the weld joint between the head plate and the shell of said vessel by virtue of the inner pressure being applied to said vessel.

2. Description of the Prior Art Recently. progress in the production in plants of the petrochemical or other chemical industries has created a need for a fresh approach to the multiple-layer shell type cylindrical pressure vessel which can withstand a considerably high inner pressure and yet is of great capacity. This has brought about the practical production of pressure vessels of this kind with substantial success and has proved that the safety of a pressure vessel of this kind is superior to that of a vessel of a solid wall type shell.

It follows then that multiple-layer shell type cylindrical pressure vessels have recently outnumbered solid wall type cylindrical vessels in the field of high presssure vessel application. However. many accidents have been experienced in the past, with high-pressure vessels such as high-pressure reactors due to the failure of the shell, particularly due to the failure of the solid wall shell type pressure vessel.

While the safety of the multiple-layer shell type pressure vessel has been proven or guaranteed by presenting a plurality of actual instances, the principle of the strength ofsuch vessel is still theoretically unknown because of the complexity of construction of a multiplelayer type shell, in contrast to the simplicity of the solid wall type shell.

In general. in case the multiple-layer shell type cylindrical vessel is used as a pressure vessel such as reactor or autoclave and thus is directly subjected to inner pressure. the stress behavior created in the vessel shell due to the inner pressure is similar to that of the vessel having a solid wall type shell. such that it is a common practice to apply the same stress calculation method to both cases. However. a close study of the stress behavior has revealed that what is important is the secondary or local bending caused in every portion of the vessel rather than the primary stress which is caused by the inner pressure. regardless of whether the vessel is of a solid wall type shell or of a multiple-layer type shell. In this respect, it is widely accepted to calculate the bending rigidity and bending force at such an edge portion of the shell which is adjacent to the weld joint between the shell and the head plate. assuming that the cylindrical vessel of the solid wall type shell can be handled in a similar manner as an ordinary bending beam.

In the case of a pressure vessel having a multiplelayer shell. the calculation of the bending rigidity or bending force at the edge of the shell may be done in a similar manner to that of the pressure cylinder of a solid wall type shell. However. in such a case, because the multiple-layer ply shell is separated at least one layer from another, the bending rigidity thereof will be smaller as compared with the solid wall shell type vessel if the ordinary calculation method is utilized. In contrast thereto, the bending force. which is created secondarily at the edge portion adjacent to the weld joint between the shell and the head plate, is associated with the bending rigidity at such edge portion. and thus the smaller the rigidity. the smaller the bending force.

The smaller the bending rigidity. the smaller the bending force. There is a consideration that the smaller bending rigidity will not necessarily result in a large bending stress, but, in practice. according to the aforesaid calculating method, a great bending stress is anticipated. It is accepted. accordingly. that the strength of the multiple-layer shell is smaller that that of the solid wall type shell.

Aside from such an assumption. there is another argument that the calculating method using a bending beam does not well represent the actual stress behavior, for the multiple-layer shell. However, there has been proposed no approach to this problem other than the above.

SUMMARY OF THE INVENTION It is accordingly a principal object of the present in vention to provide a method for increasing the strength or bending rigidity of a multiple-layer shell type cylindrical pressure vessel, particularly to increase the bending rigidity at the transition points in the bending direction of the shell.

Briefly, in accordance with one embodiment of the present invention, there is provided at at least one tran sition point in the bending-direction a circumferential welding extending throughout the wall thickness of the shell or there is provided a solid wall type wall section extending from the juncture between the head plate and the shell to the transition points or the solid wall type shell section bounded by the two transition points, thereby increasing the bending rigidity or strength of the multiple-layer shell type pressure vessel. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross-sectional view illustrative ofa multiple-layer shell type cylindrical pressure vessel having bending curvatures similar in shape to a sine-curve. with the transition points in the bending-direction shown;

FIG. 2 is a cross-sectional view of one embodiment of a pressure vessel of the type described. showing circumferential welds provided in the shell at the transition points in the bending-direction according to the present invention;

FIG. 3 and FIG. 4 are cross-sectional views of the shell of a pressure vessel using a section of solid wall type shell between the two transition points in the bending-direction so as to increase bending rigidity of a pressure vessel rather than the use of circumferential weldings; and

FIGS. 5 and 6 are explanatory views of the multiple layer shell of a pressure vessel. referring to bending curvatures and transition points in the bending direction.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawing wherein like reference characters designate identical or corresponding parts throughout the several views and more particularly to FIG. I whereon there is shown a shell of a cylindrical pressure vessel which shell is subjected to deformation or secondary bending caused by bending and shear forces at or in the vicinity of the welding joint between the shell and the head plate of the pressure vessel. As shown, there is a series of continuous bending curvatures appearing alternately on opposite sides with respect to the neutral line of the bending curvatures. As has been described, there are shown transition points in the bending-direction at 1,2,3, and The position of the first transition point in the bending-direction as viewed from the left in the figure depends on the correlation of the transition point to the weld joints with the head plates on the opposite edges of a vessel. The positions of the transition points appear at spacings ll, 12 and 13 which are claculated by the diameter, wall thickness. bending rigidity, etc. of a cylinder, regardless of the external forces.

It is known however. that the magnitude of such deformation decreases as it goes from the edge of the shell of the vessel toward the center portion thereof and is reduced to almost zero at a fairly short distance from the edge of the shell of the vessel. In other words, it is said that the influence of the external force prevailing at the edge portion of the shell of the vessel will diminish and disappear at a given distance from the edge thereof.

FIG. 5 shows a multiple-layer shell type pressure vessel which undergoes bending in one direction and thus has no transition point in the bending-direction. FIG. 6 is a view similar to FIG. 5, but the bending has two directions of curvature and hence has one transition point in the bending-direction.

For example, in the case of a high-temperature and high-pressure reactor cylinder of a diameter of 3,800 mm and a wall thickness of 200 mm, the transition points in the bending-direction appear at three points spaced 315 mm, 850 mm and 1,310 mm in the shell from the weld joint between said shell and the head plate of a pressure vessel. In the case of solid wall shell type pressure vessel of the same dimensions as those of the former, another transition point appears at a distance of 1.500 mm from the aforesaid welding joint or the edge of the shell of a vessel.

According to the present invention, circumferential weldings are provided through the wall thickness at four points, 5, 6, 7 and 8, as shown in FIG. 2, thus increasing the bending rigidity of a multiple-layer shell type pressure vessel. The reason why the first three points do not appear at the same spacings is that the second point is determined by assuming the first point being a fixed or rigid point, i.e., the rigidity at such a point is increased by providing a circumferential welding throughout the wall thickness of the shell. In other words, this span is assumed as a solid wall type shell section. The position of the third point is determined in the same manner, with the result of providing a spacing different from those of the first two points. The reason why the circumferential welding is provided at point 8 spaced 1,500 mm from the welding joint or the edge of the shell is that the span extending from the welding joint through the points 5, 6, and 7 is now assumed as a solid wall type and hence another transition point is expected to appear at point 8, as has been described earlier. Provided that there are provided at the four points 5, 6, 7 and 8 the circumferential weldings extending through the wall thickness of the shell. i.e., such span is assumed as a solid wall type, then the influence of the external force exerted on such solid wall type shell will disappear at a point spaced 1,483 mm from the edge of the shell. In other words, the influence of the shear force and bending force created at the welding joint between the shell and the head plate will disappear at the fourth point 8, thus dispensing with the fifth point.

In case such measures to increase the bending rigidity are not taken, and hence the pressure vessel has a small bending rigidity, then the point where the external force disappears will appear at a point spaced 907 mm from the edge of the shell.

Although the description has been given to the preferred embodiment of the present invention using circumferential weldings at the transition points, solid wall type shell sections may alternatively be used in place of such circumferential welds as is shown at 9 in FIG. 3. If two or more such solid wall type shell sections are continuous with each other, then there is no need to provide circumferential welding at the interface of such two shell sections. Furthermore, a substantial length of the shell portion may be replaced by the solid wall type shell section, as shown at 10 in FIG. 4.

The calculation of bending rigidity of the cylindrical wall or shell is generally in accordance with Timoschenkos calculation method (Timoschenko: Theory of Plates and Shells, A.SM.E. Code Section VII, Div. 2). In this method, the total bending rigidity of a bending beam, which has a given depth or thickness as well as a given typical radium of curvature running along the bending neutral line of the thickness. is obtained by integrating the individual rigidity of the imaginary longitudinal sections or layers included in the beam and ex tending from the inner side to the outer side of the beam. In this case, if the superposed layers maintain their relative positions, i.e. cause no relative slippage therebetween, the same bending rigidity may be obtained, whether the pressure vessel is of a solid wall type shell or a multiple-layer type shell. In other words, the multiple-layer type shell, in this case, corresponds to the case where multiple-layers maintain their mutual relative positions i.e,, cause no relative slippage. Thus, in the multiple-layer shell undergoing bending deformation, even if the shell bends at two different radii of curvatures, if such curvatures exist on the same side of the neutral line of bending, then the aforesaid relative relation with no slippage may be maintained.

Referring back to FIG. 1, the radius of curvature of the bend created between the transition point 1 and the edge of the shell adjoining the head plate appears on the upper side of the bending neutral line, and after passing through the transition point 1, it appears on the lower side of the neutral line. Thus, there results the occurance of a transition point in the bendingdirection. In such case, there results a change in the relative posi tion of one layer to another, i.e., there arises slippage between the two layers, thus reducing bending rigidity.

Accordingly, if the circumferential weld is provided at such transition point in the multiple-layer shell of a pressure vessel then the relative position with no slippage between the layers may be maintained with the result that the same assumption may be applied to the multiple-layer shell as that of the solid wall type shell. The aforesaid consideration includes the assumption that the multiple-layer type shell would not be subjected to buckling or instable failures. To prevent instable failures such as buckling, it is a common practice to use an increased thickness of the innermost layer.

As has been described. the positions of the transition points are dependent on the dimensions of the shell and head plate regardless of the levels of the inner pressure and temperature prevailing in the shell. Thus. the aforesaid measures, such as using a circumferential welding or solid wall type shell section, may be applied effectively. The embodiment of the present invention which uses a solid wall type shell section in place of the circumferential weld may avoid complexity in production, thus presenting a considerable economic saving.

It will be understood that the above description is merely illustrative of preferred embodiments of the invention. Additional modifications and improvements utilizing the discoveries of the present invention can be readily anticipated by those skilled in the art from the present disclosure, and such modifications and improvements may fairly be presumed to be within the scope and purview of the invention as defined by the claims that follow:

What is claimed as new and desired to be secured by letters Patent of the United States is:

l. A method for increasing the strength ofa multiplelayer shell type cylindrical pressure vessel having head plates, the shell of which, when inner pressure is applied to the vessel, tends to present a sine-curve like bending behavior with a transition point at the neutral axis at each curvature, the method comprising:

constructing said shell of more than one section wherein at least one of said sections is constructed of more than one layer, providing a circumferential weld between one ofsaid head plates and one of said sections, and

providing a circumferential weld extending throughout the wall thickness of said shell at a position coinciding with a first of said transition points with respect to the bending direction as viewed from said circumferential weld between said one of said head plates and said one of said sections.

2. A method as defined in claim 1, wherein at least one of said sections of said shell extending from said welding joint to one of said transition points is a solid wall type shell.

3. A method as defined in claim 1, wherein at least one section between two of said transition points is a solid wall type shell. 

1. A method for increasing the strength of a multiple-layer shell type cylindrical pressure vessel having head plates, the shell of which, when inner pressure is applied to the vessel, tends to present a sine-curve like bending behavior with a transition point at the neutral axis at each curvature, the method comprising: constructing said shell of more than one section wherein at least one of said sections is constructed of more than one layer, providing a circumferential weld between one of said head plates and one of said sections, and providing a circumferential weld extending throughout the wall thickness of said shell at a position coinciding with a first of said transition points with respect to the bending direction as viewed from said circumferential weld between said one of said head plates and said one of said sections.
 2. A method as defined in claim 1, wherein at least one of said sections of said shell extending from said welding joint to one of said transition points is a solid wall type shell.
 3. A method as defined in claim 1, wherein at least one section between two of said transition points is a solid wall type shell. 